Method for image calibration and apparatus for image acquiring

ABSTRACT

The present invention relates to a method for image calibration and an apparatus for image acquiring. In the method for image calibration, the image formation position for an image acquiring unit of the apparatus is calibrated according to the relative location of the image acquiring unit to a objective lens of the apparatus, wherein the relative location is determined by calculating the focus index of the image acquired by the image acquiring unit so that a clear and sharp interferogram can be obtained for three dimensional surface profile measuring. In addition, it is possible to obtain a clear and sharp image without any interference fringe outside the coherent range by adjusting the image formation position, which is capable of being utilized for two dimensional defect detection and dimension measurement.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to an image acquiring technologyand, more particularly, to an apparatus and a method for imagecalibration and an apparatus for image acquiring.

2. Description of the Prior Art

A white-light interferometric system cannot be used for performinglateral measurement and defect detection because interference fringesappear in the image formed at the objective focal point due to theinterferometric configuration.

To overcome such a problem, U.S. Pat. No. 7,068,376 discloses aninterferometric system as shown in FIG. 1. The optical interferometricsystem 1 comprises a light source 10, a beam splitting unit 11, aninterferometric unit 12 and an image acquiring unit 13. The light source10 generates a light beam, which becomes a collimated light beam afterit passes a collimator lens 14. The collimated light beam is reflectedby the beam splitting unit 11 to the interferometric unit 12 and thusbecomes a measurement light beam and a reference light beam. Themeasurement light beam is incident on an object to be tested 100 and isreflected thereby back to the interferometric unit 12. Then themeasurement light beam is interfered by the reference light beam to forman interfered light beam. The interfered light beam is received by theimage acquiring unit 13 after it passes the beam splitting unit 11 so asto form an image with interference fringes.

In the aforementioned prior art, a phase shift method is used tocalculate the three-dimensional profile of an object to be tested.Combined operation is performed on a sequence of images acquired by thephase shift method so as to obtain the direct-current (DC) component ofthe images. The obtained images are free of interference fringes and areused for two-dimensional lateral detection. However, the two-dimensionalfringeless images are obtained after the phase shift process, whichcauses limited imaging range and wastes longer time.

SUMMARY OF THE INVENTION

In one embodiment, the present invention provides a method for imagecalibration, comprising steps of: acquiring a sequence of interferenceimages; obtaining a sharpness focusing index curve and a contrastfocusing index curve corresponding to the sequence of interferenceimages; obtaining a peak of the sharpness focusing index curve and apeak of the contrast focusing index curve; and performing a calibratingprocess to calibrate a relative position of the peak of the sharpnessfocusing index curve to the peak of the contrast focusing index curve soas to obtain a sequence of clear interference images.

In one embodiment, the present invention provides a method for imagecalibration, comprising steps of: acquiring an image free ofinterference fringes; obtaining a sharpness focusing index curve and acontrast focusing index curve corresponding to the image; obtaining apeak of the sharpness focusing index curve and a peak of the contrastfocusing index curve; and performing a calibrating process to calibratethe relative position of the peak of the sharpness focusing index curveto the peak of the contrast focusing index curve so as to obtain a clearimage free of interference fringes.

In one embodiment, the present invention provides a method for imagecalibration, comprising steps of: acquiring an interference image;obtaining a sharpness focusing index and a contrast focusing indexcorresponding to the interference image; recording the sharpnessfocusing index and the contrast focusing index; adjusting an imageformation position of the interference image; and repeating theaforesaid steps to maximize the sharpness focusing index and thecontrast focusing index.

In one embodiment, the present invention provides an apparatus for imageacquiring, comprising: a light beam generator, capable of generating alight beam; a beam splitting unit, capable of reflecting the light beamto generate a reflected light beam; an interferoscope set, capable oftransforming the reflected light beam into a reference light beam and ameasurement light beam incident on an object to be tested and reflectedto the interferoscope set so as to combine with the reference light beamto form an interfered light beam emitted into the beam splitting unit;and an image acquiring unit, capable of receiving the interfered lightbeam from the beam splitting unit so as to acquire an image, the imageacquiring unit being coupled to a first actuator on one side.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects, spirits and advantages of the embodiments of the presentinvention will be readily understood by the accompanying drawings anddetailed descriptions, wherein:

FIG. 1 is a schematic diagram showing a conventional apparatus for imageacquiring;

FIG. 2 is a flow-chart showing a method for image calibration accordingto one embodiment of the present invention;

FIG. 3 is a schematic diagram showing an apparatus for image acquiringaccording to one embodiment of the present invention;

FIG. 4A is a flow-chart showing a method for image calibration on asequence of interference images according to one embodiment of thepresent invention;

FIG. 4B is a schematic diagram showing an image acquiring unit and aninterferoscope set according to one embodiment of the present invention;

FIG. 4C is a graph showing a sharpness focusing index curve and acontrast focusing index curve according to FIG. 4B;

FIG. 4D is a graph wherein a peak of the sharpness focusing index curveand a peak of the contrast focusing index curve are aligned;

FIG. 5A is a schematic diagram showing the relation between the imageand the image mask;

FIG. 5B is a frame histogram showing the relation between the framenumber and the intensity;

FIG. 6A is a flow-chart showing a method for image calibration on animage according to another embodiment of the present invention;

FIG. 6B is a graph showing the change of a sharpness focusing indexcurve and a contrast focusing index curve according to FIG. 6A;

FIG. 7A is a flow-chart showing a method for image calibration on animage free of interference fringes according to the present invention;

FIG. 7B is a schematic diagram showing an image acquiring unit and aninterferoscope set according FIG. 7A; and

FIG. 7C is a schematic diagram showing a range centered at the peak ofthe contrast focusing index curve.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

One embodiment of the present invention provides a method for imagecalibration, in which the focusing index is calculated to determine therelative position of the image acquiring unit to the image pick-upposition so that a clear image with/without interference fringes isobtained.

The present invention provides an apparatus for image acquiring, inwhich a charge-coupled device (CCD) of the image acquiring unit and theobjective lens position in an interferoscope set are moved so as todirectly obtain a clear image outside the coherent range for real-timetwo-dimensional dimension measurement. The clear image is free ofinterference fringes and thus can be used in auto-focusing withoutfurther processing. Therefore, the precision in auto-focusing isenhanced.

One embodiment of the present invention provides an apparatus for imageacquiring, in which a charge-coupled device (CCD) of the image acquiringunit and the objective lens position in an interferoscope set are movedso as to obtain a sequence of high contrast images within the coherentrange for three-dimensional surface profile reconstruction usingwhite-light interferometry.

One embodiment of the present invention provides a method for imagecalibration and an apparatus for image acquiring, in which the imagingquality by the interferometic system is calibrated by auto-focusing. Inthis embodiment, a clear image can be obtained inside/outside thecoherent range using a look-up table (LUT) for two-dimensional dimensionmeasurement and three-dimensional surface profile measurement withoutchanging the objective lens. Therefore, the interferometic system ofthis embodiment is two-in-one for both two-dimensional dimensionmeasurement and three-dimensional measurement.

The present invention can be exemplified by the preferred embodiment asdescribed hereinafter.

Please refer to FIG. 2, which is a flow-chart showing a method for imagecalibration according to one embodiment of the present invention. Themethod 2 for image calibration is characterized in that the focusingindex is calculated so as to determine whether the relative position ofthe image acquiring unit to the objective image pick-up position iscorrect for further calibration. The method 2 comprises steps describedhereinafter.

In Step 20, a sequence of interference images is acquired. Then in Step21, a sharpness focusing index curve and a contrast focusing index curveare obtained corresponding to the sequence of interference images. Apeak of the sharpness focusing index curve and a peak of the contrastfocusing index curve are obtained, as performed in Step 22. At last, acalibrating process is performed to calibrate a relative position of thepeak of the sharpness focusing index curve to the peak of the contrastfocusing index curve so as to obtain a sequence of clear interferenceimages.

To better understand the method for image calibration according to thepresent invention, a method for acquiring a clear image withinterference fringes by adjusting focusing index is describedhereinafter. To begin with, please refer to FIG. 3, which is a schematicdiagram showing an apparatus for image acquiring according to oneembodiment of the present invention. The apparatus 3 for image acquiringcomprises a light beam generator 30, a beam splitting unit 31, aninterferoscope set 32 and an image acquiring unit 33. The light beamgenerator 30 is capable of generating a light beam 90. The beamsplitting unit 31 is capable of reflecting the light beam 90 to generatea reflected light beam 91. The interferoscope set 32 is capable oftransforming the reflected light beam 91 into a reference light beam 92and a measurement light beam 93. The measurement light beam 93 isincident on an object 100 to be tested and is reflected to theinterferoscope set 32 so as to combine with the reference light beam 92to form an interfered light beam 94. The interfered light beam 94 isemitted into the beam splitting unit 31. The image acquiring unit 33 iscapable of receiving the interfered light beam 94 after it passes thebeam splitting unit 31 to form an image.

The image acquiring unit 33 is coupled to a first actuator 34 on oneside. The image acquiring unit 33 is a charge-coupled device (CCD) or acomplementary metal-oxide-semiconductor field-effect transistor (CMOS)device. Alternatively, the image acquiring unit 33 is a detector capableof detecting light intensity. The first actuator 34 is a motor-drivenactuator, a piezoelectric actuator or other devices capable of movingthe image acquiring unit so as to calibrate the position of the imageacquiring unit 33. Moreover, the interferoscope set 32 is coupled to asecond actuator 35 on one side. The second actuator 35 is a motor-drivenactuator or a piezoelectric actuator so as to calibrate the relativeposition of objective lens 320 to the object 100 to be tested in theinterferoscope set 32.

Please refer to FIG. 4A and FIG. 3, wherein FIG. 4A is a flow-chartshowing a method for image calibration on a sequence of interferenceimages according to one embodiment of the present invention. The method4 for image calibration comprises steps as described hereinafter.

To begin with, in Step 40, a sequence of interference images isacquired. The sequence of interference images is acquired by adjustingthe position of the image acquiring unit 33 so as to acquire thesequence of interference images in the coherent range.

In Step 41, a sharpness focusing index curve and a contrast focusingindex curve are obtained corresponding to the sequence of interferenceimages. Please refer to FIG. 4B and FIG. 4C, wherein FIG. 4B is aschematic diagram showing an image acquiring unit and an interferoscopeset that acquire the sequence of interference images and FIG. 4C is agraph showing a sharpness focusing index curve and a contrast focusingindex curve according to FIG. 4B. In FIG. 4B, 990 denotes an imagepick-up focal point of an objective lens and 991 denotes an imageformation position corresponding to 990. 7 denotes an optical systemthat comprises the light beam generator 30, the beam splitting unit 31and part of the interferoscope set 32 in FIG. 3. In one embodiment, theoptical system 7 is a microscope set. When the image acquiring unit 33is not located at a proper image formation position, for example,position 992 in the figure, the image acquiring unit 33 will acquire asequence of blur images. Meanwhile, the sharpness focusing index curveand the contrast focusing index curve are as shown in FIG. 4C.

In the present invention, the focusing index is an index for imageevaluation. The sharpness focusing index curve is an index forcalculating the difference of content values of adjacent pixels so as toevaluate the sharpness of the sequence of interference images. Thecontrast focusing index curve is an index for calculating the statisticdistribution of content values of pixels so as to evaluate the contrastof the sequence of interference images. Both the sharpness focusingindex curve and the contrast focusing index curve can be obtained byusing a spatial frequency distribution identification rule such asmulti-coefficient correlation, image differentiation, depth of peaks andvalleys, image contrast, histogram or frequency-domain analysis.

As shown in FIG. 5A, the image sharpness is calculated by performingspatial convolution of a cross-shape mask 20 and an image 21. Theconvolution sum is the sharpness value of the image 21. Moreparticularly, the mask 20 is symmetrical in shape and deploys theweighting values a1˜a8 and the weighted positions. Convolutions of theweightings on the mask 20 and the pixel values corresponding to weightedpositions on the image 21 are performed and the convolution results aresummed. Then, the same operation is performed after the mask 20 is movedto a next position where operation is to be performed. The sharpnessvalue of the image 21 is thus obtained by summing all the convolutionresults. Moreover, the sharpness value of the image 21 can also beobtained by analysis and calculation over the frequency domain.

As shown in FIG. 5B, the contrast is the width of intensity distributionof pixels, i.e., the width 16 of the frame histogram, which is thedistance between the smallest pixel value 15 a and the largest pixelvalue 15 b, filtered by a threshold value Nth to filter out the pixelvalues with pixel number below the threshold value Nth. The imageintensity is obtained by analysis and calculation of the pixelintensity, for example, the sum of the pixel intensities or the averageintensity of adjacent pixels in an image region. The image intensity isthe average luminance or the maximum luminance of the pixels in animage.

In Step 42, a peak of the sharpness focusing index curve and a peak ofthe contrast focusing index curve are obtained. Please refer to FIG. 4C,wherein FH denotes the contrast focusing index curve of the sequence ofinterference images, FT denotes the sharpness focusing index curve ofthe sequence of interference images and H denotes the peak of thecontrast focusing index curve. The peak position indicates the positionwhere the sequence of interference images appears. T denotes the peak ofthe sharpness focusing index curve. The peak position indicates theposition where the sequence of clear images appears. When theinterferometic system is correctly configured, H and T appear at thesame position where the sequence of clear interference images appears.When the interferometic system is not correctly configured, H and Tappear at different positions, as shown in FIG. 4B.

To obtain the sequence of clear interference images, Step 43 isperformed so as to set up a relation of a distance between an imageformation position and an image pick-up position to a distance betweenthe peak of the sharpness focusing index curve and the peak of thecontrast focusing index curve. To realize Step 43, the image acquiringunit 33 in FIG. 3 is moved and the change of the distance between thepeak H of the contrast focusing index curve FH and the peak T of thesharpness focusing index curve FT is observed. By repeating theaforementioned steps, the relation can be acquired.

Finally, Step 44 is performed. The distance between the image formationposition and the image pick-up position is adjusted according to therelation so that the peak position of the sharpness focusing index curveis identical to the peak position of the contrast focusing index curve,as shown in FIG. 4D. Therefore, a sequence of clear interference imagescan be acquired for three-dimensional profile reconstruction.

More particularly, to calibrate the image formation position is tocalibrate the position of the image acquiring unit 33. To calibrate theimage pick-up position is to calibrate the position of the objectivelens 320 of the interferoscope set 32. In other words, the peak H andthe peak T can be aligned so as to acquire the sequence of clearinterference images by adjusting the position of the objective lens 320or the position of the image acquiring unit 33 in the apparatus 3 forimage acquiring in FIG. 3 according to the relation obtained in Step 43.Furthermore, in Step 43, a relation table can be set up so that theposition of the image acquiring unit 33 and the position of theobjective lens 320 of the interferoscope set 32 can be automaticallycalibrated to align the peak H and the peak T according to theinformation stored in the relation table.

Please refer to FIG. 6A and FIG. 3, wherein FIG. 6A is a flow-chartshowing a method for image calibration on an image according to anotherembodiment of the present invention. The method 5 for image calibrationcomprises steps as described hereinafter.

To begin with, in Step 50, an interference image is acquired. The Step50 is similar to Step 40, thus detailed description is omitted. Then inStep 51, a sharpness focusing index and a contrast focusing index areobtained corresponding to the interference image. In Step 52, thesharpness focusing index and the contrast focusing index are recorded.In Step 53, an image formation position of the interference image isadjusted. More particularly, in Step 53, the position of the imageacquiring unit 33 is adjusted. Please refer to FIG. 6B, which is a graphshowing the change of a sharpness focusing index curve and a contrastfocusing index curve according to FIG. 6A. The aforesaid steps arerepeated to maximize the sharpness focusing index and the contrastfocusing index, as performed in Step 54, so as to acquire a clearinterference image for three-dimensional profile reconstruction usingwhite-light interferometry.

Please refer to FIG. 7A and FIG. 3, wherein FIG. 7A is a flow-chartshowing a method for image calibration on an image free of interferencefringes according to the present invention. The method 6 for imagecalibration comprises steps as described hereinafter.

To begin with, in Step 60, the position of the image acquiring unit 33and the position of the interferoscope set 32 are calibrated by movingan image pick-up focal point of the objective lens 320 of theinterferoscope set 32 to an image pick-up position 993 outside acoherent range where interference takes place in an image pick-up fieldof view so as to acquire an image free of interference fringes. Pleaserefer to FIG. 7B, in which the position of the image acquiring unit 33is calibrated by adjusting the image acquiring unit 33 to move in arange R1 around an imaging focal point 991. 7 denotes an optical systemthat comprises the light beam generator 30, the beam splitting unit 31and part of the interferoscope set 32 in FIG. 3. In one embodiment, theoptical system 7 is a microscope set. The position of the interferoscopeset 32 is adjusted by moving the image pick-up focal point of theobjective lens 320 of the interferoscope set 32 outside a coherent rangeR0 where interference takes place in an image pick-up field of view soas to acquire a clear image without interference fringes.

In Step 61, a sharpness focusing index curve and a contrast focusingindex curve corresponding to the image without interference fringes areobtained. Then in Step 62, a peak of the sharpness focusing index curveand a peak of the contrast focusing index curve are obtained. A relationof a distance between an image formation position and an image pick-upposition to a distance between the peak of the sharpness focusing indexcurve and the peak of the contrast focusing index curve is set up, asperformed in Step 63. Steps 61 to 63 are performed similarly to thepreviously disclosed, and thus detailed description thereof is omitted.

After the relation is set up, Step 64 is performed to determine a rangecorresponding to the peak position of the contrast focusing index curveas a center. Please refer to FIG. 7C, which is a schematic diagramshowing a range D centered at the peak H of the contrast focusing indexcurve FH. The size of the range D is determined according to the user'sneed.

Finally, Step 65 is performed to adjust the distance between the imageformation position and the image pick-up position according to therelation so that the peak T of the sharpness focusing index curve FT isoutside the range D so as to exhibit the clear image free ofinterference fringes for two-dimensional defect detection and dimensionmeasurement.

According to the above discussion, it is apparent that the presentinvention discloses a method for image calibration and an apparatus forimage acquiring, in which a clear image can be obtained inside/outsidethe coherent range using a look-up table (LUT) for two-dimensionaldimension measurement and three-dimensional surface profile measurementwithout changing the objective lens. Therefore, the present invention isnovel, useful and non-obvious.

Although this invention has been disclosed and illustrated withreference to particular embodiments, the principles involved aresusceptible for use in numerous other embodiments that will be apparentto persons skilled in the art. This invention is, therefore, to belimited only as indicated by the scope of the appended claims.

1. An apparatus for image acquiring, comprising: a light beam generator,capable of generating a light beam; a beam splitting unit, capable ofreflecting the light beam to generate a reflected light beam; aninterferoscope set, capable of transforming the reflected light beaminto a reference light beam and a measurement light beam incident on anobject to be tested and reflected to the interferoscope set so as tocombine with the reference light beam to form an interfered light beamemitted into the beam splitting unit; and an image acquiring unitcoupled to a first actuator on one side, capable of receiving theinterfered light beam from the beam splitting unit so as to form animage, obtaining a peak of the sharpness focusing index curve and a peakof the contract focusing index curve, and performing a calibrationprocess to calibrate a relative position of the peak of the sharpnessfocusing index curve to the peak of the contrast focusing index curve soas to obtain a sequence of clear interference images.
 2. The apparatusfor image acquiring as recited in claim 1, wherein the image acquiringunit is a charge-coupled device (CCD) or a complementarymetal-oxide-semiconductor field-effect transistor (CMOS) device.
 3. Theapparatus for image acquiring as recited in claim 1, wherein the firstactuator is a motor-driven actuator or a piezoelectric actuator so as tocalibrate the position of the image acquiring unit.
 4. The apparatus forimage acquiring as recited in claim 1, wherein the interferoscope set iscoupled to a second actuator on one side, the second actuator being amotor-driven actuator or a piezoelectric actuator so as to calibrate theposition of the interferoscope set.